The brazing of ferrous metals (e.g., carbon steel components) involves joining surfaces of ferrous metals with brazing pastes or preforms. The brazing pastes or preforms generally contains a metal or a mixture of metals and an organic (or hydrocarbon) binder. The melting point of metal or mixture of metals in brazing pastes or preforms generally is substantially lower than that of the base carbon steel components. The components are joined by juxtaposing them with the brazing paste or preform adjacent to or between them, and heating to a temperature that will effect melting of the brazing metal or mixture of metals without melting the components.
The function of organic or hydrocarbon binder is to serve as a vehicle for metal or a mixture of metals. It generally consists of pure or mixtures of low-boiling point organic or hydrocarbon compounds such as glycols and ethers. These compounds are thermally dissociated and removed from joints while heating components to be brazed to brazing temperatures.
For example, carbon steel components are generally brazed in the presence of nitrogen-based atmospheres containing controlled amounts of a reducing gas such as hydrogen and an oxidant such as moisture. The function of a reducing gas is to keep the surface of carbon steel components from oxidizing and also to maintain reducing potential in both the heating and cooling zones of the furnace. The functions of an oxidant are to help in regulating braze flow and in removing the organic binder from the braze material and to prevent formation of soot on brazed joints. The use of high concentration of a reducing gas in the atmosphere is known to cause overflow of brazing material, resulting in poor quality of brazed joints. The use of low or insufficient concentration of an oxidant is known to result in the formation of soot on brazed joints. Likewise, the use of low concentration of a reducing gas or high concentration of an oxidant is known to oxidize the braze material and components, resulting in poor braze flow and brazed joint quality and unacceptable appearance of brazed components. Therefore, it is critical to carefully select concentrations of both a reducing gas and an oxidant in the brazing atmosphere to (1) minimize overflow and underflow of braze material, (2) maintain reducing potential in the furnace, (3) assist in breaking down organic binder, and (4) prevent formation of soot on brazed joints.
The importance of controlling concentrations of hydrogen and moisture in the humidified nitrogen-hydrogen atmosphere has been described in detail in a paper titled "The Effect of Atmosphere Composition on Braze Flow" presented by Air Products and Chemicals Inc. at the 14th Annual AWSANRC Brazing and Soldering Conference held in Philadelphia, Pa. on 26-28 Apr., 1983. The teachings of this paper are incorporated here by reference.
The humidified nitrogen-hydrogen atmospheres in theory provide brazing companies ultimate freedom in terms of selecting concentrations of both the reducing gas and moisture. They also provide brazing companies ultimate flexibility in changing the overall flow rate and composition of the brazing atmosphere. However, in practice, they do not provide brazing companies means of precisely changing or controlling the concentration of moisture in the atmosphere. Often humidifiers used to add moisture in the nitrogen-based atmospheres are either too expensive or sized improperly to meet ever-changing atmosphere needs of the brazing companies. Furthermore, since a part of these atmospheres travels toward the cooling zone of the furnace and exits the furnace through the opening in the discharge vestibule, they require high concentration of hydrogen to maintain reducing potential in the cooling zone.
U.S. Pat. No. 4,450,017 discloses the use of a moisture-free nitrogen-based atmosphere for decarburize annealing carbon steels. It exposes the metal to be decarburize to a moisture-free atmosphere containing 1-50% carbon dioxide, 1-20% hydrogen, and the balance being nitrogen to a temperature close to the ferrite-austenite transition temperature of about 927.degree. C. This patent does not teach anything about (1) forming moisture in-situ in the heating zone of the furnace by reaction between hydrogen and carbon dioxide and (2) using moisture-free atmosphere for brazing carbon steels at a temperature above about 1,080.degree. C.